Prior art for dimming technology of light-emitting elements uses pulse width modulation (PWM) to convert analog signals to digital pulses, controls an on/off time ratio of light-emitting elements and then divides into some levels. Subsequently, the light-emitting elements will display the values of bright gray scale relatively. Thus, providing a width change on a digital pulse can change output current to adjust brightness of the light-emitting elements.
U.S. Pat. No. 6,989,701 discloses a PWM driving apparatus for a light emitting diode (LED) includes a saw tooth wave generator for generating a saw tooth wave signal, a comparator, a field effect transistor (FET), a first resistor, a second resistor, a power supply and an LED array. A modulation signal is provided by a modulation signal source and the saw tooth wave signal is fed to the comparator. An output of the comparator is connected to a gate terminal of the FET. The power supply is connected to a source terminal of the FET through the first resistor. A drain terminal of the FET outputs a driving current through the second resistor to the LED array.
However, the foregoing PWM driving apparatus easily causes a driving circuit of the white light-emitting elements produces a noise that the human ear can hear, because frequency of a PWM signal in the range 200 Hz to 20 kHz can be heard by the human ear. When the PWM signal is low, the driving circuit for light-emitting elements stops working. The output capacitance passes through white light-emitting elements and then discharges with the bottom resistors. Therefore, when using PWM to dim the light, the output capacitance inevitably produces large ripple. In addition, efficiency of a small duty cycle is lower when using PWM to control duty cycle.
Accordingly, a new driving circuit for light emitting elements is needed to adjust resistance of a variable resistor via a microprocessor or a mechanical method and control brightness of light emitting elements by using an analog method.